Scanning Reflection Electron Microscopy Research Articles

Structural Change of Si(001) Surfaces after Alternating Current Heating

Si(001) vicinal surfaces heated with a sine wave of 104 Hz AC are investigated by using scanning reflection electron microscopy. Surfaces of 1×2 that consist of wide 1×2 terraces (the 1×2 dimer is perpendicular to the direction of the heating current) and narrow 2×1 terraces (the 2×1 dimer is parallel to the direction of the heating current) terraces were obtained at temperatures below 850oC. At temperatures between 850oC and 1100oC, on the other hand, double-domain surfaces where 2×1 and 1×2 terraces are arranged regularly with approximately equal widths were formed. The driving force for growth changes from thermal effect to evaporation effect at about 850oC. At temperatures above 1100oC, the surfaces are composed of mosaic domains due to the evaporation of the atoms.

Oxidation of hafnium on Si(001): Silicate formation by Si migration

The oxidation behavior of 0.6 and 1.1 ML hafnium on Si(001) was studied by using scanning reflection electron microscopy with in situ x-ray photoelectron spectroscopy. The submonolayer sample was designed to purposely grow ${\mathrm{SiO}}_{2}$ on the partially exposed surfaces, and was compared with a 1.1 ML sample to discriminate the structural and chemical differences between the two samples. This approach revealed that oxygen ions were likely to form Si-O and Hf-O bonding units separately at $400\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ Torr oxygen, but at $700\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in $5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}5}$ Torr oxygen, the bonding priority of oxygen ions transforms into Hf-O-Si bonding units implying silicate formation by Si migration. Despite silicate formation, $\ensuremath{\sim}0.27\ensuremath{-}\mathrm{nm}$-thick ${\mathrm{SiO}}_{2}$-like layer is believed to have remained underneath silicate layers.

Growth of Si and Ge nanostructures on Si substrates using ultrathin SiO/sub 2/ technology

Using scanning reflection electron microscopy and a high-temperature scanning tunneling microscopy (STM), we study the growth processes of Si and Ge nanostructures on Si substrates covered with ultrathin SiO/sub 2/ films. Si windows are formed in the ultrathin SiO/sub 2/ films by irradiating focused electron beams used for SREM or field emission electron beams from STM tips before or during heating samples. Ge nanoislands are grown only at the Si window positions by depositing Ge on the samples and by subsequent annealing of them. Moreover, Ge nanoislands about 7 nm in size and ultrahigh density (>10/sup 12//cm/sup 2/) are grown on the ultrathin SiO/sub 2/ films. These nanoislands can be manipulated by STM when they are separated from Si substrates by the ultrathin SiO/sub 2/ films. Si, Ge, Ge/Si and Si/Ge/Si nanoislands can also be grown on the Si windows by selective growth using Si/sub 2/H/sub 6/ and GeH/sub 4/ gases. These nanoislands are found to be stable on the Si windows during high-temperature annealing. These results indicate that ultrathin SiO/sub 2/ technology is useful for growing Si and Ge nanostructures on given areas.

Effect of oxygen pressure on the structure and thermal stability of ultrathin Al2O3 films on Si(001)

Al 2 O 3 /Si(001) surfaces and interfaces were investigated using scanning reflection electron microscopy, reflection high-energy electron diffraction, x-ray photoelectron spectroscopy, and Auger electron spectroscopy. A uniform, stoichiometric and ultrathin Al2O3 film of about 0.6 nm was grown on an atomically flat Si(001)-2×1 surface, and the resulting Al2O3/Si(001) interface was atomically abrupt. An intentional reoxidation of the Al2O3/Si(001) system under low oxygen pressure (2×10−6, 5×10−6, and 2×10−5 Torr O2) showed that the ultrathin Al2O3 film stoichiometry and the interface abruptness were maintained with progress in reoxidation time. Furthermore, the film and the interface showed no degradation under low-pressure reoxidation at various temperatures (400–750 °C). A high-pressure reoxidation of the Al2O3/Si(001) system at 5×10−5 Torr O2 resulted in the formation of an interfacial SiO2 layer which grew in a layer-by-layer mode with atomic-scale uniformity and had an atomically abrupt interface with Si(001) substrate up to 700 °C. Additionally, a very weak temperature dependence of the growth of interfacial SiO2 was observed. A high-pressure reoxidation at 750 °C led to the formation of crystalline ultrathin Al2O3 film and also caused degradation of the film by formation of SiO2 in the near-surface region, where a slight decrease in the Al2O3 film thickness was observed. This was attributed to the formation of interstitial Si in the interfacial SiO2 layer and the subsequent mobility of Si and Al under this growth condition. Under low-pressure reoxidation, the Si and Al were immobile because of the absence of an interfacial SiO2 layer at the Al2O3/Si(001) interface. These results indicate that the oxygen pressure of the ambience plays an important role in the oxidation of the Al2O3/Si(001) interface, and the mobility, transport, and chemical reactions at various oxidation temperatures (400–750 °C).

Selective thermal desorption of ultrathin aluminum oxide layers induced by electron beams

The mechanism of electron-beam-induced selective thermal desorption of ultrathin aluminum-oxide layer (∼0.4 nm) on Si(001) surface was investigated by using scanning reflection electron microscopy, reflection high-energy electron diffraction, and Auger electron spectroscopy. We found that the change in the aluminum-oxide layer composition induced by electron-stimulated oxygen desorption accounted for the selective thermal desorption of the oxide layer. A systematic increase in the vacuum-annealing temperature to 500 °C, 600 °C and 720 °C resulted in the formation of three-dimensional metal aluminum clusters, desorption of these clusters, and creation of a nanometer-scale clean Si(001)-2×1 open window in the selected electron-beam-irradiated area.

Study of ultrathin Al2O3/Si(001) interfaces by using scanning reflection electron microscopy and x-ray photoelectron spectroscopy

Al 2 O 3 /Si (001) interfaces were investigated using scanning reflection electron microscopy and x-ray photoelectron spectroscopy. A uniform and stoichiometric ultrathin Al2O3 film of ∼0.6 nm was grown on an atomically flat Si(001)-2×1 surface, and the resulting Al2O3/Si(001) interface was atomically abrupt. Furthermore, an intentional high-pressure oxidation shows that we can grow Si oxide at the Al2O3/Si(001) interface with atomic-scale uniformity as this oxidation proceeds in a layer-by-layer manner. The resulting Si oxide/Si(001) interface was also atomically abrupt. In addition, the rate of oxidation of Si at the Al2O3/Si(001) interface depends strongly on the O2 pressure.

Scanning reflection electron microscopy study of surface defects in GaN films formed by epitaxial lateral overgrowth

We have used scanning reflection electron microscopy (SREM) to detect surface defects in GaN films formed by facet-initiated epitaxial lateral overgrowth. SREM revealed individual threading dislocations and single atomic steps on the GaN surface, and provided images of crystallographic tilting near the surfaces. We found that one of the two tilted GaN crystals in the overgrown areas became dominant and that the surface changed to a single domain after 50-μm-thick GaN deposition. Our SREM results also showed that the deposition of thick (over 100 μm) GaN films significantly improves the crystallographic structures of the overgrown regions, and reduces the threading dislocations in the GaN films.

Mechanism of Layer-by-Layer Oxidation of Si(001) Surfaces by Two-Dimensional Oxide-Island Nucleation at SiO2/Si Interfaces

We have studied the mechanism of layer-by-layer oxidation of Si(001) surfaces. The layer-by-layer oxidation was confirmed and precisely monitored by scanning reflection electron microscopy (SREM). By combining SREM and scanning tunneling microscopy (STM) methods, we investigated the change in atomic-scale roughness at SiO2/Si(001) interfaces during the oxidation. We found that, while the oxide interface is atomically flat after the oxidation of each layer is complete, nanometer-scale oxide islands with a single atomic height are densely nucleated at the interface during the oxidation of each layer. We also observed an oscillation in the intensity of reflection high-energy electron diffraction (RHEED) spots during the top-layer oxidation. These results clearly indicate that the layer-by-layer oxidation proceeds by the nucleation of nanometer-scale oxide islands at the interfaces and by their preferential lateral island growth.

Characteristics of Fielo-GaN Grown by Hydride Vapor Phase Epitaxy

ABSTRACTThe crystal quality of facet-initiated epitaxial lateral overgrowth (FIELO) -GaN, in particular, that in the vicinity of the GaN surface, is reported. It is shown that the surface smoothness of FIELO-GaN enables us to use it as an “epi-ready” substrate. The crystallinity of FIELO-GaN is evaluated by x-ray rocking curve (XRC) measurements. We indicate that the FWHM of XRC should be reduced with the decrease of the dislocation density. We previously reported that the dislocation behavior of FIELO-GaN causes the tilting of the c-axis in the overgrown regions. By using scanning reflection electron microscopy (SREM), however, we show that the tilting on the surface of thick FIELO-GaN was negligibly small.

Observation and nucleation control of Ge nanoislands on Si(111) surfaces using scanning reflection electron microscopy

Using a high-resolution scanning reflection electron microscope with multifunctions, we investigate Ge nucleation processes on clean Si(111) surfaces and form Ge islands on them by controlling step arrangements of Si(111) surfaces and by using focused electron beam (EB)-induced surface reactions. It is found that three-dimensional (3D) Ge islands grow selectively at step band areas on the surfaces without growth of the islands at terrace areas. Three-dimensional Ge nanoislands are formed at given points by stimulating the Ge wetting layer using focused electron beams and scanning tunnelling microscopy. Ge nanoislands are also formed by depositing Ge on Si windows in ultrathin SiO2 films and subsequent annealing of the sample. The islands are formed only at the window positions. These results imply new methods for forming Ge quantum dots or nanostructures at given areas.

Observation and control of Si surface and interface processes for nanostructure formation by scanning reflection electron microscopy

We observe the oxidation process on clean Si surfaces using high-resolution scanning reflection electron microscopy and form nanostructures on them using focused electron-beam- (EB-) induced surface reactions. Si thermal oxidation occurs layer by layer, and the interface between the oxide film (<1 nm thickness) and Si substrate becomes atomically abrupt. When the sample is heated to 700-800 °C after being irradiated by the focused EB on the surface at room temperature, SiO2 film is selectively decomposed from the EB-irradiated area, resulting in the exposure of a clean Si substrate. The typical width of the clean Si `open windows' is about 10 nm. Using selective reactions during heating after the deposition of Si and Ge films on the patterned samples, Si nanowires and Ge nanoislands with 10 nm size are formed on Si surfaces. Ga-adsorbed Si nanoareas and Ga nanodots are also formed by selective adsorption of Ga on the Si window areas.

Initial oxide-growth process on the Si(100) surface

We investigate the initial growth of oxide on the Si(100) surface by means of first-principles molecular-dynamics calculations. By inserting oxygen atoms successively into the backbonds and the dimer bonds, the variation in surface geometry and in adsorption energy is studied. Once a backbond is oxidized, substitution at the rest backbond of the dimer atom gets much less favorable. The other bonds show a similar reactivity to that on the clean surface. Thus, the oxide growth of the first atomic layer proceeds rather uniformly until one monolayer coverage. This process seems compatible with the recent observation of layer-by-layer oxidation by scanning reflection electron microscopy [H. Watanabe et al., Phys. Rev. Lett. 80 (1998) 345]. However, the energetics of the second-layer oxidation lead to preferential inward growth rather than lateral growth.

Simultaneous observation of scanning tunneling microscopy and reflection electron microscopy image of the Si(111)7×7 surface

Abstract Atomic structures of scanning tunneling microscopy (STM) tips were characterized by reflection electron microscopy (REM). The Si(111)7×7 surface was observed simultaneously by REM and STM in an ultrahigh vacuum electron microscope. In high-resolution REM images of the specular beam reflection, a tungsten tip apex was observed with its mirror image. The distance between the true and the mirror images was used to estimate the tip–surface distance. As the tip scanned over the Si(111) surface in STM experiments, the tip apex was scraped to give a plateau of a (110) atomic surface. An atomic-resolution STM image of the Si(111)7×7 surface was obtained when the STM tip apex had a single atom on the (110) terrace.

Selective growth of nanometer-scale Ga dots on Si(111) surface windows formed in an ultrathinSiO2film

Selective growth of nanometer-scale Ga dots on patterned ultrathin ${\mathrm{SiO}}_{2}$ films was studied by using scanning-reflection electron microscopy and energy-dispersive x-ray spectroscopy (EDX). Nanometer-scale Si(111) surface windows were fabricated by electron-beam-induced thermal decomposition of the film. Ga was deposited on the patterned surfaces at room temperature to 550 $\ifmmode^\circ\else\textdegree\fi{}$C. Under certain deposition and annealing conditions, Ga dots were present only on the Si(111) surface windows, and the smallest size of the dots was about 20 nm. To understand the selective growth of Ga dots, we measured the desorption rate and the surface-diffusion length of Ga atoms until all atoms desorbed from the ${\mathrm{SiO}}_{2}$ surface and nucleated forming random dots. The EDX measurement showed that the desorption rate from Ga dots on ${\mathrm{SiO}}_{2}$ films was 2 to 2.5 times larger than that on Si(111) surfaces, and that the activation energy of desorption rate from ${\mathrm{SiO}}_{2}$ films was 1.33 eV. The Ga surface-diffusion length was estimated by measuring the temperature dependence of the Ga depleted zone width near the linear Si surface windows. The surface-diffusion length of Ga atoms on ultrathin ${\mathrm{SiO}}_{2}$ films increased when the substrate temperature was increased. Thus, we were able to selectively grow Ga dots on only the Si(111) surface windows.

Instability of 2D Ge layer near the transition to 3D islands on Si (111)

The formation of a stable surface morphology of Ge on Si(111) at coverages close to the transition from two-dimensional (2D) to threedimensional (3D) growth was studied using scanning reflection electron microscopy and energy dispersive X-ray spectroscopy. Regions of the stable surface morphologies of 2D layers and 3D islands are described by a phase diagram as a function of coverage and temperature. Temperatures were determined at which the unstable 2D Ge layer at coverages between 1.5 and 2.3 BL does not transform into islands longer than 10 min. It was found that irradiation by focused e-beam creates points in the unstable 2D layer where 3D islands appear after annealing. These 3D islands probably nucleate on structural defects introduced by the irradiation. This shows that the instability of the 2D layer can be used for controllable nucleation of islands.

Layer-by-Layer Oxidation of Si Surfaces.

Layer-by-layer oxidation of Si surfaces has been studied by scanning reflection electron microscopy, which allowed us to obtain plan-view images of buried SiO2/Si interfaces. We observed atomic steps at the SiO2/Si(111) interfaces and the periodic reversal of terrace contrast during oxidation of Si(001) surfaces. We found that the initial surface structure was preserved at the interface and that the interfacial steps did not move laterally during oxidation. These results mean that layer-by-layer oxidation is governed by random nucleation of nanometer-scale oxide islands and lateral island growth. We also studied the kinetics of initial layer-by-layer oxidation of Si(001) surfaces. Our results showed that barrier-less oxidation of the first sub-surface layer, as well as oxygen chemisorption onto the top-layer, occurred at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3eV. The initial oxidation kinetics is discussed based on first-principles calculations.

Layer-By-Layer Oxidation of Silicon Surfaces

ABSTRACTLayer-by-layer oxidation of Si(111) and (001) surfaces has been studied by using scanning reflection electron microscopy (SREM). We found that SREM images reveal interfacial structures of the SiO2/Si system. Our results showed that the initial step structure of Si substrates was preserved at SiO2/Si interfaces and that interfacial steps did not move laterally during oxidation. We also observed a periodic reversal of terrace contrast in SREM images during the initial oxidation of Si(001) surfaces. These results indicate layer-by-layer oxidation of Si surfaces, which is promoted by the nucleation of nanometer-scale oxide islands at SiO2/Si interfaces. In addition, we investigated the kinetics of initial layer-by-layer oxidation of Si(001) surfaces. We found that a barrierless oxidation of the first subsurface layer, as well as oxygen chemisorption onto the top layer, occur at room temperature. The energy barrier of the second-layer oxidation was found to be 0.3 eV. The initial oxidation kinetics are discussed based on first-principles calculations. Moreover, we confirmed that the layer-by-layer oxidation of Si surfaces holds true for conventional furnace oxidation.

HF-chemical etching of the oxide layer near a SiO2/Si(111) interface

The HF-chemical etching process near the SiO2/Si(111) interface (&amp;lt;1 nm) is investigated by scanning reflection electron microscopy and microprobe Auger electron spectroscopy. The HF etching of the SiO2 layer thermally grown on an atomically flat Si(111)-7×7 surface progresses in a layer-by-layer manner, and then the final layer of oxide (∼0.3 nm) is removed in a two-dimensional void expansion with the H-terminated Si surface. This very uniform HF etching is thought to reflect the structural anisotropy of the SiO2 layer formed near the SiO2/Si(111) interface.

Atomic-scale structure ofSiO2/Siinterface formed by furnace oxidation

${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}$ interfaces formed by furnace oxidation are investigated by scanning reflection electron microscopy (SREM). SREM observations reveal that the initial atomic steps on the Si(111)-7\ifmmode\times\else\texttimes\fi{}7 and Si(001)-2\ifmmode\times\else\texttimes\fi{}1 surfaces are preserved at the ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}$ interfaces and the interfacial atomic steps do not move laterally during furnace oxidation. A profile analysis of reflection high-energy electron diffraction indicates that the atomic-scale roughness at the ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}$ interfaces is formed by furnace oxidation. The respective ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}(111)$ and ${\mathrm{SiO}}_{2}/\mathrm{S}\mathrm{i}(001)$ interfaces are made up of about 5- and 3-nm-diam islands. Our results indicate that the layer-by-layer oxidation caused by two-dimensional island nucleation progresses during furnace oxidation.

Atom technology project: Recent activities

The “Atom Technology” project started in fiscal 1992 as one of MITI’s 10 year projects, aims at systematically establishing technology for handling individual atoms and molecules on a solid surface or in a three-dimensional space, as a generic technology for various fields of industry. This project, closely adjacent to science, emphasizes the following three key focuses: atom manipulation, nanoscale self-organization, and critical-state phase control, with two basic approaches of in situ dynamical observation (experimental) as well as ab initio calculation (theoretical). In this article, several topics were picked up from recent activities at the Joint Research Center for Atom Technology (JRCAT) for Phase I of the initial 6 years (1992–1997) and some technical details were described: (1) ultrathin SiO2 on Si(001) surfaces; layer-by-layer oxidation, its kinetics, scanning reflection electron microscopy observation, and scanning tunneling microscopy observation of leakage sites; (2) growth and transport of structure-controlled SinHx+ clusters for deposition using a novel ion trap; and (3) colossal magnetoresistance and related phenomena in perovskite-type manganese oxides. Research plans for phase II (1998–2001) of the project will be also touched upon.